My youngest who is now 11 was very withdrawn when she first started school. We're in the UK and she was very young in her year. She was just 4. She did not talk to the teacher. Her teachers in her first 2 years were very concerned about how quiet and withdrawn she was especially as out of school she was not withdrawn. Her teacher in Yr 1 helped out at a church weekend away and saw her in a different environment where she was not withdrawn and was amazed. I think she just found school hard work. Not the work so much as the expectations and the social side. The good news is although she remained quiet at school she got a lot more confident sfter the age of 7. In fact last year (year 6) I was told she was positively chatty. She has just matured and she blossomed academically eventually. It took her to Year 6 to catch up. She is now in secondary school and although she doesn't love school and would always prefer to be at home, she is fine. She has lots of friends, gets invited out and is doing well academically. When you are young it is hard being at school. Just have patience.

We recently went through this with our 6 year old. He loves school, but started giving me a hard time about going. I talked with his teacher and she said everything was fine in class and when he was there he had a great time. We talked with our son and he just kept saying when he is in school, he feels like he is sick. But he would never go to the school nurse. It took about 3 weeks of this (every day!). Finally, after trying to talk to him and understand what is going on, we figured out that he sits with an older boy on the bus in the morning who is bullying him and others. I spoke with the bus driver, he rearranged some seats, spoke with the bully, and my son has been more than happy to go to school ever since!
So look at all aspects and don't give up! By helping her figure out what's up and how to fix it, you'll help her start to enjoy school.
Good luck!!